1. Field of the Invention
The present invention relates to a luminescent material. Further, the present invention relates to a light-emitting element having a pair of electrodes, a layer including a luminescent material that can emit light by being applied with electric field. Moreover, the present invention relates to a light-emitting device having such a light-emitting element.
2. Description of the Related Art
A light-emitting element using a luminescent material has advantages of thinness, lightness in weight, fast response, direct-current low-voltage driving, and so on, and is expected to be applied to a next-generation flat panel display. Further, a light-emitting device having light-emitting elements arranged in matrix is superior to a conventional liquid crystal display device in a wide viewing angle and high visibility.
A light-emitting element has the following light-emission mechanism; voltage is applied to a light-emitting layer sandwiched between a pair of electrodes, electrons injected from a cathode and holes injected from an anode are recombined in a light-emission center of the light-emitting layer to form molecular excitons, and then light is emitted by releasing energy when the molecular exciton returns to the ground state. As the excited state, a singlet-excited state and a triplet-excited state are known, and the light emission is possible by either of the excited states.
Emission wavelength of a light-emitting element is determined by energy difference between a ground state and an excited state, i.e., a band gap, of a light-emitting molecule included in the light-emitting element. Therefore, various emission colors can be obtained by devising structures of the light-emitting molecules. By forming a light-emitting device using light-emitting elements capable of emitting red light, blue light and green light, which are light's three primary colors, a full-color light-emitting device can be manufactured.
However, there is a problem of such a full-color light-emitting device. The problem is that formation of a light-emitting element having excellent color purity is not easy. This is because it is difficult to realize a light-emitting element with high reliability and excellent color purity, although light-emitting elements for red, blue and green with excellent color purity are needed so as to manufacture a light-emitting device having superior color reproducibility. As a result of recent development of materials, light-emitting elements for red and green have achieved high reliability and excellent color purity. However, in particular, a light-emitting element for blue cannot be realized high reliability and excellent color purity.